Mortgage Extra Payment Calculator Widget

Show homeowners what overpaying really buys them. Visitors enter the balance, rate and years left, then an extra amount every month, once a year or as a single lump sum, and see the new payoff month, the time and interest saved and a balance curve with and without the extras.

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<iframe src="https://a2z.tools/embed/w/mortgage-extra-payment-calculator" title="Mortgage Extra Payment Calculator by A2Z Tools" width="100%" height="980" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.

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How it works

First the regular payment is found from the standard level-payment formula for the balance, the annual rate divided by twelve and the remaining months. The widget then replays the loan twice, month by month. The first replay pays only the regular amount; the second also takes the extra monthly sum, the yearly extra on every twelfth payment and the lump sum on the payment number you choose, and sends all of it straight to principal. Each month interest is charged on the balance left after the previous payment, so every dollar of overpayment stops attracting interest for the rest of the term. The difference between the two replays gives the months saved and the interest saved, and the chart plots both balances year by year. Entering the date of the next payment turns the month count into a calendar payoff month. A lump sum scheduled after the loan would already be repaid is rejected.

Calculation method

  • Regular payment = P x r / (1 - (1 + r)^-n), r = annual rate / 12, n = remaining months
  • Each month: interest = balance x r; balance = balance + interest - payment - extra
  • Extra = extra monthly + (extra yearly on payments 12, 24, 36 ...) + (lump sum on the chosen payment)
  • Months saved = months without extras - months with extras
  • Interest saved = total interest without extras - total interest with extras

Worked examples

Adding $200 a month

Inputs: $200,000 balance, 6%, 30 years left, $200 extra every month

Result: Paid off in 21 years instead of 30; interest saved $79,800.51

The regular payment is $1,199.10; total interest falls from $231,676.38 to $151,875.87.

A one-time bonus

Inputs: $200,000 balance, 6%, 30 years, $20,000 lump sum with payment 60

Result: New payoff 24 years 9 months; 5 years 3 months and $56,099.58 of interest saved

The lump sum lands when the balance is still high, so it removes the interest on $20,000 for the next 21 years.

An illustration for planning, not financial advice. Your lender's statement and terms take precedence.

Limitations

  • Assumes a fixed rate for the whole remaining term; an adjustable-rate loan will reset and change both the payment and the saving.
  • Escrow for property tax and insurance, mortgage insurance and any prepayment charge are left out.
  • Interest is computed monthly on the balance; loans with daily interest or biweekly payment plans give slightly different figures.

Where publishers use it

  • A mortgage broker's article on whether to overpay or invest spare cash
  • A personal-finance blog post about paying off a house before retirement
  • A bank's landing page explaining its penalty-free overpayment allowance
  • A financial planner's client portal for testing what a yearly bonus does to the loan
  • A first-time buyer guide showing the effect of rounding the payment up

Questions

Why does a small monthly overpayment save so much?

Every extra dollar cuts the principal immediately, so it is never charged interest again for the remaining life of the loan. On $200,000 at 6% over 30 years, adding $200 a month removes 9 years and about $79,800 of interest, because early in the loan most of each regular payment is interest.

Is a lump sum better than the same money spread monthly?

Money paid earlier saves more, because it stops interest sooner. A $20,000 lump sum in payment 60 of a $200,000 6% loan saves about $56,100 and 5 years 3 months. The same $20,000 spread thinly over later years saves less. Compare both in the widget with your own figures.

Does the regular payment change after I overpay?

Not in this calculator - it keeps the contractual payment and lets the loan finish early, which is how most lenders treat overpayments. Some lenders instead recalculate (recast) a lower payment over the original term; ask yours which applies.

What if my lender charges an early repayment fee?

The widget does not include fees. Many fixed-rate loans outside the US allow only a set percentage of the balance to be overpaid each year without a charge, for example 10%. Subtract any charge from the interest saved before deciding.

How do I find the balance and the months remaining?

Your latest statement shows the outstanding principal and usually the maturity date. Count the payments between your next due date and the maturity date; 27 years and 4 months is 328 payments, or 27.33 years in the term box.

Can I enter a 0% loan?

Yes. At 0% the payment is simply the balance divided by the months, there is no interest to save, and the widget shows only how much sooner the loan ends.

Sources

  1. A Consumer's Guide to Mortgage Refinancings - Board of Governors of the Federal Reserve System . Worked figures used to check the payment formula: a $200,000 loan for 30 years at 6% costs $1,199 a month and $231,640 in interest. Checked 2026-10-01.

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Mortgage Extra Payment Calculator
https://a2z.tools/embed/mortgage-extra-payment-calculator

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